Image device



June 23, 1959 vR. K. ORTHUBER ETAL VlvmGE DEVICE 2 Sheets-Sheet 1 FiledDeo. 2, 1955 I S v mE w mw C r/. K Rc O B A w EP .0. E NT E mw F n. .en.w 0 wm l :v /ow C n wm Ill.....LI TJ......w50m r U. Pm EE f 0 I. Wc 9 rww f 4. w T l P um 8 X .3m x m P n u SL .--ZT uw m n n JM U L f M m 9. 87 6 5 4 3 2 IJ 0 NEQncuEE H RICHARD K. ORTHUBER CHRISTIAN C. LARSONEXCITING y@ @iff 2,892,095 MAGE DEVICE Richard KasparOrthuber-j andChristian Charles Larson, Fort Wayne, Ind., assignors to InternationalTelephone and Telegraph Corporation Original application August 16,'1955, Serial No. 528,700. Divided and this application December 2,1955, Serial No. 550,665

s claims. (crass-213) This is a divisional application ofOrthuber-Larson application Serial No. 528,700 led August 16, 1955 andentitled, Image Device.

The present invention relates to an image device, and more particularlyto a solid-state laminated cell which is capable of reproducing invisible form a radiation image.

There is disclosed `in Ullery application Serial No. 362,204, tiled June17, 1953, now Patent No. 2,773,992, and entitled, Display Amplier andMethod of Making Same, a display-amplifying device which is capable ofreproducing or intensifying a radiation image. This 'device is alaminated structure composed of electroluminescent phosphor andphotosensitive layers sandwiched between two plate-like electrodes. Asource of alternating voltage is coupled to these two electrodes, Vandthe impedance of the photosensitive layer inthe absence of radiation isdesigned to be so high that such voltage will not cause the phosphorlayer to luminesce. However, in the presence of exciting radiation thevimpedance of the `photosensitive layer is reduced suiciently to impressa greater amount of voltage over-the phosphor layer, thereby causing itto luminesce.

In the Ullery application, the photosensitive layer is particularlydesigned to provide the necessary vimpedance control for regulating thevoltage applied to the phosphor layer. This photosensitive layer in oneform consisted of a base panel of quartz or fotoform glass having aplurality of spaced transverse apertures which were lined respectivelywith evaporated photoconductive material such as cadmium sulphide. Thephotoconductive material lining theapertures constituted theimpedance-changing portion of the display device and served to controlthe degree of excitation of the adjoining phosphor material.

This invention is intended to constitute an improvement over the deviceof the earlier application and to provide a slightly different mode 4of.operation which utilizes regeneration in the display device itself forintenstifying-the reproduced image. Y

vlt is an object of this invention to yprovide a method and apparatusfor intensifying an image.

It is an object of this invention to provide a method for .utilizingfeedback radiation forintensifying a `reproduced image. 4

It is still another object to provide a solid-state display device whichutilizes feedback between the phosphor and photosensitive layers in sucha manner as to `provide a faithful reproduction of a given image, Whichreproduction includes shades of gray as well as extreme highlight andlowlight levels.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will `be--best understood by reference to the followingdescription of an embodiment of the invention taken irl-conjunction withthe accompanying drawings wherein:

Fig. l is a cross-sectional view in diagram lform of a display device ofthis invention;

'ice

Fig. 2 is a graph used in explaining the operation of .this invention; j

Fig. 3 is an enlarged fragmentary cross-section of one` specicembodimentof this invention; Y

Fig. 4 is an enlarged section of the photosensitive layer of Fig. 3; K y

Fig. 5 is a cross-section of a cathode ray device utilizing the cell ofFig. 3; A v A Fig. 6 is a block diagram of a circuit ilkwhich includesthe tube of Fig. 6; y y x i Fig. 7 is an illlustration of Wave formsused in explaining the operation of the circutof Fig.` 6; and

Fig. 8 is a diagrammatic illustration of a switching device used inconjunction with-a laminated cell of slightly different constructionthan that shown in Fig. 3.

Referring to Fig. l of the drawings, the display device or laminatedcell is indicated by the reference numeral -1.- The laminations of thiscell comprise a glass o'r the like supporting plate 2, a transparentfilm of conductive material 3, such as evaporated silver or a layerresulting from a reaction of stannous `chloride with the glass plate(NESA) applied to one side of plate 2, a lamina of photoconductivematerial .4 (cadmium sulphide, for example) applied to the iilm 3, alayer of electroluminescentphosphor material 5 mounted on the layer 4,another film 6 of conductive material on the phosphor lamina 5 and asupporting glass plate 7 which may fbe applied to the lm 6. A lightattenuating insulating layer (not shown in Fig. l) may be interposedbetween layers 4 and 5 for limiting light feedback therebetween.

A source of alternating voltage such as 600 volts at 400 cycles isconnected to the films or plates 3 and 6. Assuming no incident radiationon the photosensitive layer 4, the impedance of this layer is designedto he so high that an insuicient portion of the yapplied voltage willappear over the phosphor layer 5 itself to cause excitation thereof.However, with incident radiation on the layer 4, the impedance isreduced suliciently to cause an increase over the phosphor layer 5 to anextent suiiicient to cause luminescence thereof. By properly designingthe impedances of the two layers 4 and 5, the phosphor layer 5 may hemade to luminesce with a brightness` corresponding to incident radiationfallingon the photosensitive layer 4. The fundamentals of operation of adisplay ydevice of this character are fully explained in theaforementioned Ullery application as well as in published literature. Y

When the phosphor layer 5 is luminescing, ,a certain amount of thisluminescence is returned to` the photoconductive layer 4. This returned`or feedback light causes a further change in the impedance of the layer4, thereby causing further excitation of the phosphor layer 5. lf thisaction is allowed to continue, a saturation condition will be reached at.which the phosphor will continue to luminesce and lthe feedback lightfalling on the photoconductive layer will Yserve to hold this saturationcondition.

This feedback or regenerative action is readily Yunderstood by referencetothe graph of Fig. 2. This graph presents the response curve of atypical display device or screen and plots light flux emitted per unitarea by the phosphor as a function of photoconductor illumination. Thecurves are calculated by assuming (l)V the brightness of theelectroluminescent panel increases with the third power of the appliedalternating current voltage; (2) the photoconductive lamina responds tothe illumination in a linear fashion; (3) the capacitance of thephotoconductive lamina is about 20% of the capacitance of an equal areaof the adjacent electrolumineseent lamina; and (4) the power factor ofthe electroluminescent lamina is negligibly small (e.g., below .15).

The absci'ssa represents incident illumination on the photoconductor inmillilumens per square centimeter.

The ordinate represents phosphor brightness output also in terms ofmillilumens per square centimeter. The S- curve 8 is the display screenresponse curve with no lightfeedback while thestraight line 9 representsthe illumination of the photoconductive layer 4 caused by the phosphorradiation with approximately 10% of the light of the phosphor layerbeing fed back thereto.

It is evident that once the phosphor 5 has been excited to approximately8.5 units brightness (point which requires an illumination on thephotoconductive layer of approximately .75 unit, this illumination canbe provided by feedback between the phosphor and photoconductor,whereupon the phosphor will maintain this brightness without furtherincident radiation falling on the photoconductor. By means of thisregeneration or feedback, the cell has driven itself to equilibriumbrightness which is a stable condition. A similar stable condition orequilibrium brightness point can be seen to exist at point 11 where thetwo curves 8 and 9 cross. Still a third equilibrium point is found atthe intersection 12 of the feedback line 9 and the response curve 8.This point 12, however, is unstable, but its importance resides in thefact that initial excitation to a point somewhat below three-tenths(0.3) unit will cause the phosphor brightness to drift down to the lowerbrightness point 11. Excitation to a point above point 12 will cause thebrightness to build up until the upper stable equilibrium point 10 isreached.

Saturation brightness corresponding to point 10 will be maintainedindefinitely as long as the excitation voltage is applied to the screen.In order to extinguish the screen itself, it is necessary to momentarilyreduce or cut olf the exciting voltage to such an extent that the gainof the screen drops so much that the upper equilibrium points 12 and 10disappear (dotted curve in Fig. 2).

These operating features as characterized by the graph of Fig. 2 can beutilized in different ways to obtain either images having no half-tones(shades of gray) or images which constitute faithful reproductions ingray scale of a given image.

In Figs. 3, 4 and 5 are shown one form of display screen which utilizesoptical feedback for intensifying an image. With reference to thesegures, the screen consists of a glass plate 13 which carries atransparent, conductive lm 14 of any suitable material such as stannousoxide. On top of this film 14 is an electroluminesccnt phosphor lamina15 which is in contact with a sheet of perforated glass 16. This glasspreferably consists of conventional fotoform glass which can be etchedphotographically in closely controlled patterns to provide a pluralityof spaced apertures 18. The upper surface of the perforated glass sheetcarries a metallic coating 19 which may be applied by evaporation or thelike. During this evaporation, care must be exercised to prevent metalfrom depositing on the inner walls of the apertures 18. This is done bylling the holes with a ne powder of glass, chalk or magnesium oxide orsimilar nely comminuted powder just prior to evaporation of the metal.After evaporation, this powder is shaken or blown out of the apertures.u

Photoconductive material, such as cadmium sulphide, is evaporated fromthe underside of the glass sheet 16 onto the walls of the apertures 18.The cadmium sulphide deposits on the underside of the glass sheetsubsequently are removed by grinding or the like. Thus, each aperture 18is provided with a photoconductive lining which is in contact at itsupper end with the metallic gating 19 and at its lower end with thephosphor layer In one form of the invention, the glass sheet 16 isopaque to light which emanates from the phosphor layer 15. By thisarrangement, the feedback path of the light between the phosphor andphotoconductive layers is so restricted that light emitted from onelphosphor elemental area can reach only the associated but not thelaterally adjoining elements of the photoconductor.

It will now be apparent that each aperture 18 together with the adjacentelemental area of phosphor 15 forms an element of the display screen,which may, provided the sensitivity or response of both thephotoconductor and phosphor elements are high enough, provide gains`higher than the light feedback between the phosphor and photoconductor.When these conditions prevail, the screen will reproduce a given imagewith brightness output corresponding to the saturation point 10 of Fig.2.

The screen of Fig. 3 is mounted in the front end of the cathode ray tube21 of Fig. 5 which is conventional as a television picture tube in everyrespect. The apertures 18 open inwardly so that the electron beam 22 mayimpinge the photoconductive linings 20. The beam 22 is scanned over thescreen in the usual manner in accordance with conventional televisionstandards, deflection coils 23 denoting the necessary deflecting meansfor accomplishing this function. As the electron beam strikes, forexample, the photoconductive lining 20 of one aperture 18, the impedanceof this respective lining is reduced by bombardment-induced conductivitythereby causing an increase of exciting potential over the adjacentphosphor element 15. Thus a beam of uniform intensity scanned over thedisplay screen at the usual frequencies will cause the entire panel toluminesce. By utilizing feedback from the phosphor, the panel willluminesce with saturation brightness.

It will be noted that the intensifier screen of Figs. 3 and 5 does notcarry any kind of a supporting plate on top of the glass sheet 16. By soconstructing the screen, it can be mounted in the evacuated envelope ofa cathode ray tube as shown in Fig. 5, the individual photoconductivelinings 2.0 being thereby exposed to bombardment by a scanning beam 22.

Assuming for the moment that the time constant of the photoconductor 20is extremely short as compared to the frame time of the televisionpicture being produced, the brightness of each element would adjustitself to the upper or lower equilibrium brightness levels 10 or 11,respectively, within a small fraction of the frametime. Therefore, theaverage brightness observed during a frame would be substantially eitherthe upper or lower equilibrium levels 10 or 11 and intermediate tones(shades of gray) would be completely absent. This lack in grayscale-reproduction can be remedied if the photoconductor used respondsto changes in illumination with a time-constant comparable toframe-time. This condition may be accomplished by either varying thechemical composition of the photoconductive material 20 to obtain theproper time-constant or by varying the scanning or frame-time to renderthe time constant of the photoconductor either equal to or longer thanthe frame-time. With the time constant so adjusted, the rate with whichany elemental area of the display screen approaches an equilibrium valuewill be so slow that such equilibrium brightness will never be quitereached during the period of one frame. The brightness average,therefore, over the period of one frame will depend on the level ofinitial excitation, or stating the same in other Words, the imagereproduced by the phosphor will contain shades of gray corresponding toshades of gray included in the exciting image.

Now having shown that it is possible to reproduce images without grayscale on the one hand or on the other hand images which do contain grayscale, attention is now directed to the problem of terminatingregeneration just prior to the development of saturation brightness. Ithas already been mentioned that saturation brightness can beextinguished by interrupting or reducing the exciting voltagemomentarily to a suiciently low level that will provide inadequateexciting voltage for the phosphor. However, for the reproduction of atelevision picture, restoration of the screen to its unexcited statemust be "accomplished oncein eachV frame. "Thus,v"a's' seen in but thatduring the retrace period' "25,"theexcitin`g voltage to the screen ismomentarily substantially reduced or t turnedotf completely to returnthe screen to itsfunexcited or darkenedv state. This control ofrthescreen may' be accomplished by means of the apparatus shown in Fig. 6

wherein like numerals indicate like parts. Suitable scan-V`ningfcircuits 26 .of conventional design supply the necessary scanningsignals (Fig. 7a) to the dellecting coils 23, these same scanningsignals being fed to a conventional blanking circuit 27 which producessuitable blanking pulses 28 during each retrace 25. The exciting voltagefor the display screen 1 is furnished by an audio frequency generator 29which may be switched on and off by means of the blanking pulse 28. Asatisfactory mode of operation is to use the pulse 28 to bias thegenerator 29 to cut olf, thereby effectively switching off completelythe exciting voltage to the display screen 1.

Thus it is seen that for each active period of scan 24, which representsthe period of one picture frame, exciting voltage is applied to thedisplay screen 1, but that during the retrace intervals 25, the excitingvoltage tothe screen 1 is turned olf. During the active scan period 24the screen 1 can approach saturation brightness through its regenerativeaction, but because of the relatively slow time constant of thephotoconductor, this saturation brightness will never quite be reached.

While reproduction of an intensied television image is possible throughthe use of the system of Fig. 6, excessive vertical shading resultssince the lower parts of the screen have much less time available todrift or regenerate toward the upper stable Ibrightness condition andwould also 4be presented to the observer for a much shorter period oftime, thus decreasing the apparent brightness in the lower parts of thepicture. Therefore, the screen should not Ibe de-energizedsimultaneously in its entire area but only in elemental areas such assingle horizontal lines or a group of horizontal lines simultaneously.

In Fig. 8 is illustrated a display screen which is substantiallyidentical to that of Fig. 3 with the exception that the conductive lm onthe phosphor layer 15 is actually composed of a plurality of horizontal,mutually insulated, parallel extending conductive strips 30 which areindividually connected to a mechanical rotary switch or distributor 31so that at any moment only one of the strips 30 is disconnected from theexciting voltage source 32. The switch consists of a plurality of statorcontacts indicated by the arrows and a rotor contact 33 which makes onecomplete cycle in the same period of a television picture frame.Rotation of the rotor 33 is synchronized with the electron beam suchthat the disconnected strip 38 is just one step ahead of the strip beingscanned by the beam.

By this switching means, exciting voltage to the elemental areas of thedisplay screen 1 is sequentially interrupted, thereby preventing thevertical shading eifect previously mentioned.

An alternative method of returning the display screen 1 to its unexcitedcondition consists in avoiding initial incident excitation above theunstable equilibrium point corresponding to numeral 12 of Fig. 2. Thiswould mean that the incident illumination or excitation on thephotosensitive layer 4 should be something less than .3 unit. In thisevent, the regenerative eifect will cause the brightness output of thephosphor to drift downwardly toward he lower equilibrium point 11.Therefore half-tone reproduction is possible without the necessity ofdeenergizing the display screen or parts thereof by a switchingprocedure as illustrated by Fig. 8.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by exhibiting the property` of electron`bombardment-induced conductivity, said photosensitive panel comprising abase of insulating material having a plurality of transverse apertures,photosensitive material lining .the walls'of said V'apertures,"aconductive material `on the surface of said base remote from saidphosphor panel, said conductive material making electrical contact withthe adjacent portions of said photosensitive lining, said apertures withthe lining and circumscribing conductive material being physically openand thereby accessible to electron bombardment, a plurality of spacedparallel strips on the phosphor panel surface remote from saidphotosensitive panel, said strips being mutually insulated from eachother, and means for applying an exciting potential between saidconductive material and said strips individually.

2. A feedback image-reproducing device comprising a panel ofelectroluminescent phosphor material, a panel of photosensitive materialadjacent to and electrically coupled to said phosphor panel, saidphotosensitive material exhibiting the property of electronbombardment-induced conductivity, said photosensitive panel comprising aIbase of insulating material having a plurality of transverseapertures,photosensitive material lining the walls of said apertures, aconductive material on the surface of said base remote from saidphosphor panel, said conductive material making electrical contact withthe adjacent portions of said photosensitive lining, said aperture withthe lining and circumscribing conductive material being physically openand thereby accessible to electron bombardment, a plurality of spacedparallel strips on the phosphor panel surface remote from saidphotosensitive panel, said strips being mutually insulated from eachother, and switching means for applying an exciting potential betweensaid conductive material and said strips individually, said switchingmeans including contacts which apply said exciting potential to saidstrips in orderly sequence.

3. An image-reproducing apparatus comprising a cathode ray tube havingan electron gun and a target, means for scanning the beam of said gunover said target, said target comprising a panel of electroluminescentphosphor material, a panel of photosensitive material adjacent to andelectrically coupled to said phosphor panel, said photosensitivematerial exhibiting the property of electron bombardment-inducedconductivity, said photosensitive panel comprising a base of insulatingmaterial having a plurality of transverse apertures, photosensitivematerial lining the walls of said apertures, a conductive material onthe surface of said base remote from said phosphor panel, saidconductive material making electrical contact with the adjacent portionsof said photosensitive lining, said apertures with the lining andcircumscribing conductive material being physically open and therebyaccessible to electron bombardment, a plurality of spaced parallelstrips on the phosphor panel surface remote from said photosensitivepanel, said strips being mutually insulated from each other, andswitching means for applying an exciting potential between saidconductive material and said strips individually, said switching meansincluding y contacts which apply said exciting potential to said stripsin orderly sequence.

4. A feedback image-reproducing device comprising a panel ofelectroluminescent phosphor material, a panel of photosensitive materialadjacent to and electrically coupled to said phosphor panel, saidphotosensitive material exhibiting the property of electronbombardmentinduced conductivity, said photosensitive panel comprising abase of insulating material having a plurality of transverse apertures,photosensitive material lining the lswitching means includescircuitryfor sequentially applying in a given order an excitingpotential between said conductive material and said elementsindividually.

References Cited in the le of this patent UNrTEDsTATEs PATENTS 2,728,815Kaum Dec. 27, 195s 2,773,992 Uuery Dec. 11, 1956 FOREIGN PATENTS 157,101Australia June 16, 1954

